Forms of Porphyra linearis (linear and broad) from Nova Scotia were evaluated for conspecificity as were forms of Porphyra purpurea (linear and broad). Morphological characteristics were determined for the forms and for other Porphyra species from the area. Conchocelis/conchosporangia cultures were initiated and maintained. DNA was extracted from the Porphyra isolates from the area and the ITS1 and ITS2 regions of the ribosomal cistron, as well as the 5.8S gene, 3$\sp\prime$ flanking region of the 18S gene and 5$\sp\prime$ flanking portion of the 28S gene, were amplified as a single unit. The resulting PCR products were sequenced directly on an ABI sequencer. The sequences were aligned using Clustal V and phylogenetically analyzed using PAUP 3.1 and PHYLIP DNAPARS. The P. linearis forms did not present consistent morphological differences. The sequences for the ITS1 and ITS2 regions were highly divergent and the forms were more closely related to other Porphyra species than to each other. Parsimony analysisof the 5.8S genes concurred with the spacer sequences in separating the two forms phylogenetically. The P. purpurea forms presented clear morphological differences in the following areas: shape (broad vs. linear); colour (dark purple vs. pale brown or pale purple); and plastid number (tendency toward displastidial cells vs. monoplastidial). Phylogenetic analysis (ITS1, IT2 and 5.8S) suggested that the forms were no more related to each other than to other Porphyra species. In particular, the linear form of P. purpurea consistently grouped with P. umbilicalis and the broad form of P. linearis. (Abstract shortened by UMI.)

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